Superconductive r.f. linear particle accelerator section having a scalloped tubular shape



May 26, 1970 A. ELDREDGE 3,514,652

SUPERCONDUCTIVE R.F. LINEAR PARTICLE ACCELERATOR SECTION HAVING A SCALLOPED TUBULAR SHAPE Filed Dec. 22, 1967 2 Sheets-Sheet 1 /-'5 FIG. I

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I NVEN'TOR.

ARNOLD L. ELDREDGE ATTORNEY A. L. ELDREDGE May 26, 1970 SUPERCONDUCTIVE R.F. LINEAR PARTICLE ACCELERATOR SECTION HAVING A SCALLOPED TUBULAR SHAPE 2 Sheets-Sheet 2 Filed Dec. 22, 1967 \I m E R Hm o 0 D M1 R ll-NAN TIRMTI CL m 5 A SE 0w 6 E NV 0c NW E HA s NOAI M A RRL m wm E d H l l m nUD NW ATI EL IL rr.

E M CL S S A INVENTOR- ARNOLD L. ELDREDGE ATTORNEY United States Patent O 3,514,662 SUPERCONDUCTIVE R.F. LINEAR PARTICLE AC- CELERATOR SECTION HAVING A SCALLOPED TUBULAR SHAPE Arnold L. Eldredge, Woodside, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Dec. 22, 1967, Ser. No. 692,896 Int. Cl. H01j 25/10, 25/34; H01v 11/00 U.S. Cl. 315-35 3 Claims ABSTRACT OF THE DISCLOSURE A superconductive microwave linear particle accelerator is disclosed. The particle accelerator structure includes a superconductive microwave accelerator section comprised of a plurality of superconductive coupled cavities arranged in a linear path and energized with microwave energy from a suitable source such as a high power klystron for accelerating charged particles passable through the microwave structure to nearly the velocity of light. The microwave accelerator structure is formed by a tube of superconductive material such as niobium or lead having its side wall scallop-contoured to define a plurality of axially spaced coupled cavity resonators for interaction with the beam passable therethrough. The accelerator structure is immersed in a cryogenic fluid such as liquid helium at approximately 2 K. to render the coupled cavity structure superconductive to minimize microwave energy losses within the accelerator structure.

DESCRIPTION OF THE PRIOR ART Heretofore, superconductive microwave linear particle accelerators have been constructed utilizing a microwave coupled cavity accelerator section made of a superconductive material. The coupled cavity structure was of the disc-loaded waveguide type wherein a plurality of washershaped discs are periodically spaced within a hollow cylindrical tube to define a plurality of successively axially spaced coupled cavities for interaction with a beam of charged partciles for accelerating same to nearly the velocity of light. When the discs and tubular waveguide structure are formed of a suitable superconductive material such as niobium or lead, a problem arises in removing the heat generated at the perimeter of the beam holes in the discs. Superconductive material such as lead and niobium have rather poor thermal conductivity and, thus, removal of the heat from the slow wave structure is a serious problem limiting the power handling capability of the accelerator.

Attempts have been made to improve the cooling of the disc-loaded waveguide structure by plating the superconductive material upon a copper disc-loaded waveguide structure such thatthe heat generated on the discs is conducted from the superconductive material to the copper substrate and thence through the copper substrate to the cryogenic fluid. Such an arrangement has the problem that the thermal coeflicient of expansion of the copper and the superconductive material is not the same such that thermal stresses are produced in the material which deleteriously alfect the Q of the accelerator section.

In another previously proposed disc-loaded accelerator section, cooling of the superconductive structure is obtained by plating the superconductive material upon the interior surfaces of a disc-loaded waveguide type substrate member made of a porous material which is permeated by superfluid helium for cooling. Such a structure is described and claimed in co-pending U.S. application 632,415 filed Apr. 20, 1967 and assigned to the same assignee as the present invention. A problem with 3,514,662 Patented May 26, 1970 "ice such a structure is that the plating of the superconductive material upon the interior surfaces of the disc-loaded substrate member is relatively difficult to achieve in practice.

Therefore, a need exists for a microwave superconductive accelerator section which will have improved heat transfer from the interior surfaces of the structure to the liquid helium in which the structure is immersed and which does not require that the interior surfaces be formed by plating.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved superconductive microwave accelerator section and methods of fabricating same.

One feature of the present invention is the provision of a microwave linear particle accelerator section formed by a relatively thin tubular member of superconductive material having its side walls scallop-contoured to define a plurality of axially spaced cavity resonators for interaction with a beam of charged particles axially passable therethrough for accelerating the particles to nearly the velocity of light, whereby substantially uniform heat transfer is obtained over the entire accelerator structure from the inside of the accelerator structure to the cryogenic liquid in which it is immersed.

Another feature of the present invention is the same as the preceding feature wherein the scallop-contours define ellipsoidal-shaped cavity resonators within the accelerator section with the major axes of said ellipsoidalshaped cavities being disposed perpendicular to the longitudinal axis of the contoured tube.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing, partly in block diagram form, of a microwave linear accelerator incorporating features of the present invention,

FIG. 2 is an enlarged sectional view of a portion of the structure of FIG. 1 delineated by line 22,

FIG. 3 is cross-sectional view of a portion of the structure of FIG. 2 taken along line 3-3 in the direction of the arrows,

FIG. 4 is a fragmentary elevational view depicting the step of inserting a mandrel into a tube of superconductive material,

FIG. 5 is a schematic diagram depicting forming of a superconductive tube to the scallop-contour of a mandrel disposed inside of the tube,

FIG. 6 is a flow-diagram, partly in block diagram form, and partly in section, depicting the steps of removing the mandrel from the scallop-contoured superconductive tube, and the treatment of the tube to obtain a suitable linear accelerator section,

FIG. 7 is a longitudinal sectional view of an alternative accelerator section similar to that of FIG. 2,

FIG. 8 is a fragmentary longitudinal sectional view of a superconductive tube formed to the interior scallopcontoured surface of a mandrel, and

FIG. 9 is a sectional view of the structure of FIG. 8 taken along line 9-9 in the direction of the arrows- DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a linear particle accelerator 1 incorporating features of the present invention. The accelerator 1 includes a source of 2 of charged particles to be accelerated to nearly the velocity of light. Such charged particles may be electrons or ions.

The source 2 formsand projects a beam of charged particle s {such as electron s, axially through an elongated tubular accelerating section 3 for interaction with the microwave fields of the accelerator 3. A high vacuum pump 4 is connected in gas communication with the accelerator section 3 for evacuating same to a low pressure as of 10- Torr. A source of high power microwave energy such as klystron 5 is coupled to the accelerator section 3for exciting same. A resistive termination 6 is connected tothe output end of the accelerator section 3 for absorbing remnant microwave energy which is unused by the accelerator section 3. A high energy particle permeable window member 7 such as a thin sheet of aluminum foil closes off the output end of the accelerator section 3 to hold off atmospheric pressure and to permit the vacuum to be maintained within the accelerator section 3 while permitting the beam of charged particles to emerge from the accelerator section 3 through the window 7.

[The accelerator section 3 is immersed in a cryogenic fluid operating at cryogenic temperatures, such as, for ex ample, liquid helium, operating at approximately 2 K., contained within a cryostat 8 surrounding the elongated accelerator section 3. A solenoid 9 surrounds the cryostat 8 for producing an axially directed beam-confining magnetic field which focuses the beam of charged particles through the accelerator section 3.

The accelerator section 3 is electrically equivalent to a disc-loaded waveguide structure conventionally used for linear accelerator structures. The accelerator structure is formed by a tube of superconducting material such as niobium or lead. The tube has its side wall scallop-contoured to define a plurality of successive ellipsoidal-shaped .cavity resonators 12 capacitively coupled together by means of a multitude of axially aligned beam holes, 14 in the end walls of the cavities defined by the inwardly directed portions of the scallop-contoured tube. A first buncher section of the accelerator structure 3 has its individual cavities dimensioned such that the excited fields of the resonators tend to bunch the charged particles of the beam into tight bunches which pass into a subsequent section of the accelerator section 3, wherein the cavities are dimensioned and spaced different from the first cavities to primarily accelerate the bunches of charged particles to nearly the velocity of light. The accelerated beam particles emerge from the accelerator section through window 7 in the form of a pulsed beam of tightly bunched beam particles. I Referring now to FIGS. 2 and 3, the accelerator structure 3, with its surrounding cryostat, is more clearly shown. The scallop-contoured tube 11, as of 0.060" to 0.200" wall thickness for accelerators operating within the band from X-band to L-band, has a maximum diameter varying from one inch at X-band to 3 inches at L- band. The scallop-contoured tube 11 defines a plurality of ellipsoidal-shaped cavity resonators 12 capacitively coupled together by means of beam coupling holes 14. The end walls of the cavities 12 are not parallel and therefore the tendency for these cavities to multifactor is reduced. Furthermore, the relativelyrounded nose portions defining the perimeter of the beam holes 14 tend to reduce the peak field strength for the microwave fields for a given energy gradient in the accelerator.

the Q of the operating structure. The cryostat 8 includes Moreover, the scallop-contour of the superconductive tube 11, vwhich defines the cavities 12, facilitates heat transfer from the inside walls of the cavities to the helium contained within 7 the, surrounding cryostat 8 in which the accelerator structure 3 is immersed. Furthermore, the scallop-contour of the accelerator structure 3 greatly minimizes the number of joints such as brazed joints, welded joints, diffusion joints, or the like, utilized to fabricate the. structure. Therefore, the microwave currents flowing inthe walls of the structure do not have to cross a great number of joints in the superconductive tube 11. Such .joints which have heretofore been encountered in discloaded waveguide structures tend to be lQssy and to reduce an inner cylindrical chamber 15 containing the liquid helium at approximately 2 K. A hollow cylindrical evacuated chamber 16 surrounds the inner chamber and a liquid nitrogen filled chamber 17 surrounds the evacuated chamber 16.

Referring now to FIG. 4, there is shown a first step in a method for fabricating the scallop-contoured accelerator structure 3. In this method, a mandrel 21, as of 6061 aluminum, has its outside surface scallop-contoured in conformance with the inside dimensions of the cavity resonators to be formed in the accelerator structure 3. The mandrel 21 is slipped into a relatively thin walled tube 22 of superconducting material as of niobium or lead. The grain size of the superconductive material forming the tube 22 should be relatively small as of less than 0.025 to provide a cold workable material, i.e., readily deformable in the cold state without producing fracture of the material.

In FIG. 5, the tube 22 is clamped to the mandrel 21 as indicated by arrows 23, and the mandrel is spun about its longitudinal axis while a forming tool 24 presses the tube 22 into the scallop-contours of the mandrel 21. The resultant tubular member 22, as formed onto the mandrel, is shown in FIG. 6. As an alternative to spinning the mandrel and employing the forming tool 24, the tube 22 may be hydraulically pressed into the contoured surface of the mandrel 21, as indicated by the annular hydroforming channel 25, which is sealed at its inner diameter to the outside diameter of the tube 22 via O-rings 26 disposed opposite successive peaks 27 and 28 of the mandrel. As an alternative to use of O-rings 26 for sealing the hydro-forming chamber 25 to the tube 22, a rubber diaphragm or bag may be employed which is expandable from the inside of the channel 25 into the depressions in the mandrel 21. The hydro-forming channel 25 is then successively moved down the length of the mandrel to form the tube 22 into all of the successive depressions in the mandrel 21.

Referring now to FIG. 6, there is shown a flow-diagram, partly in block diagram form, depicting removal of the mandrel from the scalloped tube 22 and treatment of the resultant scallop-contoured tubular structure to form an accelerator section. In step (b), the mandrel 21 is etched out of the scallop-contoured tube 22 by means of a sodium hydroxide etching bath which attacks the 6061 aluminum without appreciably reacting with or dissolving the tube 22. In step (c), the resultant scallopcontoured tube 22 is annealed by raising its temperature to 2500 C. for a time period of 2 hours in a vacuum atmosphere to substantially increase the size of the grains and to relieve stresses produced in the tube 22 by the cold Working step. The Q of the resultant structure is preferably as high as possible, and this means that the grain size should be as large as possible. Moreover, the resultant structures should be free of mechanical stress. After annealing, in step (d), the inside surface of the scallop-contoured tube 22 is electropolished to remove any surface irregularities and whiskers that might otherwise tend to lower the Q and increase the power dissipation Within the resultant accelerator structure. The re sultant structure is then excited with microwave energy at its frequency of operation and the structure is rough tuned to approximately the proper frequency by removing material from the inside walls of at least one of the resonators or by deforming the walls of one or more of the cavity resonators 12 formed in the structure. After the rough tuning, the structure is excited with microwave energy and fine-tuned by deforming the walls of at least one of the cavities. If necessary, the annealing step may be repeated after the first electropolish and tune step to remove any stress produced in the structure by the tuning operation. The resultant tuned and polished structure is then assembled in step (e) to form the completed accelerator 1 as shown in FIG. 1.

A niobium accelerator section having large grain size, and the method for annealing the niobium to grow large grains is disclosed and claimed in copending US. application 692,491, filed Dec. 21, 1967, and assigned to the same assignee as the present invention.

Referring now to FIG. 7, there is shown an alternative accelerator structure 3' similar to that shown in FIG. 2 with the exception that the accelerator structure 3 is of a bi-periodic type inasmuch as the structure is made of a succession of halfand full-sized ellipsoidal-shaped cavities 12 and 12 respectively. Such a structure is electrically equivalent to and approximates the bi-periodic accelerator structure described by J. N. Weaver et al. in an article entitled Accelerating Structures for Superconducting Linacs, appearing in the Transaction on Nuclear Science, June 1967, pp. 345-349. One of the advantages of the bi-periodic accelerator section 3' is that the cavities can be formed in relatively short sections and joined by means of joints 31 provided within the half-length cavities 12'. In such a case, the joints 21 do not substantially interfere with the strong circulating currents in the combined accelerator section due to the weak fields for the accelerating 1r/2 mode found in the half-size cavities whereby the Q of the accelerator struture 3' for the 1r/2 mode is not substantially reduced due to the joints 31. Such a biaperiodic structure would be operated in a resonant 1r/2 mode with the microwave power coupled into the accelerator structure near its center most cavity.

Referring now to FIGS. 8 and 9 there is shown an alternative method for fabricating scallop-contoured accelerator sections of the present invention. In this case, the mandrel 33 includes a central axial bore having the interior surfaces of the bore provided with a scallopcontoured surface corresponding to the outer surface of the accelerator section to be formed. In other words the mandrel 33 includes a plurality of ellipsoidal-shaped cavities interconnected by longitudinal bore. The tube to be formed with the scallop-contoured surface is inserted within the longitudinal bore of the mandrel 33 and hydraulic pressure or a forming tool is inserted within the tube 11 for expanding the tube 11 outwardly into the contours of the mandrel. After the tube 11 of superconductive material 11 has been formed into the contour of the internal bore of the mandrel 33, the mandrel is separated by removing bolts 34 which hold the two halves of the mandrel 33 together. When the mandrel is separated, the resultant accelerator section may be readily removed from the mandrel. The resultant accelerator section is then preferably annealed, electropolished and then assembled according to steps (c) through (c) of the method depicted in FIG. 6.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a superconductive microwave linear accelerator section, means forming a relatively thin hollow tube of superconductive material having its side wall contoured with a plurality of axially spaced inwardly directed scallops to define an array of axially spaced end walls for a succession of axially spaced radio frequency coupled resonator chambers defined within the interior of said tube in the region between successive scallops, and the inside diameter of said contoured tube at said cavity end wall portions defining a plurality of axially aligned beam holes.

2. The apparatus of claim 1 including, means for immersing said contoured tube in a cryogenic fluid for cooling said tube to cryogenic temperatures, means for exciting said cavity resonators with microwave energy, and means for projecting a beam of charged particles through said axially aligned beam holes for interaction with the fields of said cavity resonators for accelerating the particles of the beam to nearly the velocity of light.

3. The apparatus of claim 1 wherein said scallop-contours of said tube are shaped to define ellipsoidal shapes for said cavity resonators with the major axes of said ellipsoidal shaped cavities being perpendicular to the longitudinal axis of said contoured tube.

References Cited UNITED STATES PATENTS 3,080,527 3/ 1963 Chester. 3,103,454 9/1963 Scapple et al. 29-600X 3,290,762 12/1966 Ayuzawa et a1. 29600 HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner US. Cl. X.R. 

